1. Field of the Invention
The present invention relates to an improved process and apparatus for the bending of sheets of glass.
Such products are commonly used as windows for motor vehicles in general, and in particular for automobiles. This kind of use requires that the sheets of glass be manufactured respecting precisely both the prescribed dimensions and tolerances and the configuration required by the body of the automobile. It is furthermore necessary that the visibility through the sheets of glass is not obstructed by optical distortions or defects in the windows.
Tempering treatment using heat increases the resistance of the sheets of glass to impact, and guarantees that the fragments of glass produced in the case of breakage will be less dangerous than fragments which would be produced in non-tempered glass.
2. Description of the Prior Art
As is known, sheets of tempered glass, in particular those used by the automobile industry, can be produced in plants with a horizontal line of travel, permitting a high production throughput.
In these plants, each sheet is transported horizontally through a tunnel oven in which it is heated up to a deformation temperature, is then transferred to a bending station where the sheet is shaped, and then reaches a tempering station and successively a cooling station.
With reference to the bending station, a bending method is known which uses the force of gravity and the pressure of a mould in order to obtain the shaping of the sheet of glass in the most exact manner possible.
In particular, the pressure-bending of the heated sheets takes place between male and female moulding plates having complementary shaping surfaces.
The sheets of glass are horizontally supported by a female plate, hereinafter referred to as the "ring", the bearing surface of which is divided into segments to allow the passage of the ring itself upwards by means of a plurality of conveyor rollers.
Descriptions of this prior art can be found in U.S. Pat. No. 3,338,695 (Ritter), in French patent No. 1,474,251 (LOF), in U.S. Pat. No. 3,684,473 (Ritter) and in Luxembourg patent No. 65146 (Ritter). The method used to bend the sheets of glass according to the above mentioned patents consists essentially in the fact that the sheet itself, after having been heated in an oven up to the softening temperature, arrives, by means of horizontal conveyor rollers, in registry with the shaping ring, which, passing beyond the conveyor rollers, lifts up the sheet and carries it towards the mould, wherein the final shaping takes place. Subsequently the ring is lowered, passing back through the conveyor rollers and leaving the curved sheet of glass on the same horizontal rollers. The convey rollers then carry the sheet of glass towards the tempering station.
During the past years great commercial importance has been given to the production of curved and tempered sheets of glass with more and more reduced thicknesses (going from values of between 5 and 6 mm in thickness to values of between 2.5 and 4 mm in thickness), with ever greater dimensions, due to the increases in the window surfaces in automobiles, and having complex curvatures, in particular spherical curvatures.
In practice, the sheets of glass have become thinner, larger and preferably spherical. The above-described shaping technology is no longer suitable for the production of sheets of glass according to the market requirements.
In addition to the above, a further difficulty in production is the market trend toward requiring sheets of glass which are curved to conform to the bodywork, these sheets thus presenting wings which are sharply curved towards the interior of the vehicle.
The prior art method is not suitable for providing this kind of production at a commercially acceptable cost, as the prior art method is not capable of an efficient control of the curvature of the glass. The glass is of a thickness ranging from 2.5 to 4 mm and tends to increase more than is necessary its concavity towards its bottom, due to the effect of the force of gravity. Furthermore, the prior art is not capable of checking the precise shape of the sheet itself, especially when the sheet has sharp upwardly-turned wings, because when the sheet is laid down on the conveyor rollers it tends to lose the shape which has been given to it, as the sheet is still in a plastic state. The object of the present invention is to overcome the above mentioned problems, providing an improved process and apparatus for the bending of sheets of glass which permits the bending of sheets of glass into complex forms having reduced thicknesses and large dimensions.